Organic solar cells (OSCs), also known as organic photovoltaic (OPV) devices, have the most different device architectures. Typically, they comprise at least one organic semiconducting layer that is arranged between two electrodes. The organic layer can be a blend of a donor and an acceptor such as P3HT (poly3-hexyl-tiophene) and PCBM (phenyl C61 Butyric Acid Methyl Ester). Such simple device structures only achieve reasonably efficiencies if interfacial injection layers are used to facilitate charge carrier injection/extraction (Liao et al., Appl. Phys. Lett., 2008.92: p. 173303). Other organic solar cells have multi-layer structures, sometimes even hybrid polymer and small molecule structures. Also tandem or multi-unit stacks are known (see US 2007/090371 A1, or Ameri, et al., Energy & Env. Science, 2009.2: p. 347). Multi-layer devices can be easier optimized since different layers can comprise different chemical compounds (or simply compounds) and their mixtures which are suitable for different functions. Typical functional layers are transport layers, photoactive layers, injection layers, etc.
Optically active compounds are compounds with a high absorption coefficient, for at least a certain wavelength range of the solar spectra, which compounds convert absorbed photons into excitons which excitons in turn contribute to the photocurrent. The photoactive compounds are typically used in a donor-acceptor heterojunction, where at least one of the donor or the acceptor is the light absorbing compound. The interface of the donor-acceptor heterojunction is responsible for separating the generated excitons into charge carriers. The heterojunction can be a bulk-heterojunction (a blend), or a flat (also called planar) heterojunction, additional layers can also be provided (Hong et al, J. Appl. Phys., 2009.106: p. 064511).
The loss by recombination must be minimized for high efficiency OPV devices. Therefore, the compounds in the heterojunction must have high charge carrier mobilities and high exciton diffusion lengths. The excitons have to be separated into charge carriers at the heterointerface and the charge carriers have to leave the optically active region before any recombination takes place. For that reasons, currently, fullerenes (C60, C70, PCBM, and so on) are the preferred choice as acceptor materials in OPV devices.
Transport compounds for opto-electronic devices are required to be transparent, at least in the wavelengths wherein the device is active, and have good semiconducting properties. These semiconducting properties are intrinsic, such as energy levels or mobility, or extrinsic, such as charge carrier density. The charge carrier density can also be extrinsically influenced by doping the compound with an electrical dopant.
OSCs very often require the use of at least one n-dopant in an n-doped electron transport layer, or as a pure interlayer promoting electron injection from a conductive layer into a semiconductor or from a semiconductor into another semiconductor.
Several different n-dopants are known, such as Tetrakis(1,3,4,6,7,8-Hexahydro-2H-pyrimido [1,2-a]pyrimidinato)ditungsten (II) from EP 1 768 200 B1, Bis(2,2′-terpyridin)ruthenium, and others. One main problem of n-dopants is that since they are strong donors, they easily degrade by reacting with atmospheric oxygen. There are not many known compounds which are able to directly work as n-dopants which are also air stable. Precursor-compounds were developed with the aim to provide air stable organic compounds and being able to work as n-dopants, examples of such precursors are disclosed in WO 2007/107306 A1.
Also, only a few organic compounds are known to be able to efficiently dope low LUMO compounds used in OSCs, such as fullerenes (e.g. C60) or fullerene derivatives (e.g. PCBM), for example the compounds disclosed in US 2007/145355 A1.